Method and apparatus for producing and directing an electron beam



MTRO4 June 17, 1969 c, w, HANKs ETAL METHOD AND APPARATUS FOR PRODUCINGAND DIRECTING AN ELECTRON BEAM Sheet 7 Filed May 22, 1967 lNVENTOiZCmaas FAA 15 Hon/420 E RNEY5 June 17, 1969 w, HANKS ET AL 3,450,824

METHOD AND APPARATUS FOR PRODUCING AND DIRECTING AN ELECTRON BEAM FiledMay 22. 1967 Sheet 2 of 2 INVENTOES 639421.55 A flax/V115 Homea P. Mmz'e A AAA/AA M4 #41, m, M

AT TOR NEY5 United States Patent Ofice 3,450,824 Patented June 17, 19693,450,824 METHOD AND APPARATUS FOR PRODUCING AND DIRECTING AN ELECTRONBEAM Charles W. Hanks, Orinda, and Howard R. Harker, Mill Valley,Califi, assignors, by mesne assignments, to Air Reduction Company,Incorporated, a corporation of New York Filed May 22, 1967, Ser. No.640,138 Int. Cl. H01j 37/30 US. Cl. 13-31 6 Claims ABSTRACT OF THEDISCLOSURE Apparatus for producing and directing an electron beam, whichincorporates a transverse magnetic field for deflecting the beam throughan arcuate path, the electrons toward the outer edge of the arcuateportion of the beam for the greater part of their path of travel passingthrough a weaker region of the field relative to the electrons towardthe inner edge of the arcuate portion of the beam.

The present invention relates to apparatus for producing and directingan electron beam and more particularly to apparatus for focusing such anelectron beam.

Electron beam furnace systems utilize one or more electron beam gunassemblies for producing and directing high energy electron beams to atarget for heating the same. Electron beam gun assemblies generallycomprise an electron source or emitter for emitting electrons, andsuitable means for focusing the electrons into a beam. The beam ofelectrons is accelerated along aninitial path by a suitable acceleratinganode. Magnetic fields may be provided to direct the electron beamthrough a desired path onto the surface of a target in the furnace andto focus the beam to a desired concentration.

One type of electron beam gun assembly which offers particularadvantages utilizes magnetic fields extending transverse to thedirectionof travel of the electrons in the beam to deflect the beam of electronsthrough a curving path onto the target. Deflection of the electron beamby transverse fields enables the electron source to be positioned out ofa line of sight of the target. Thus the electron source is not directlyexposed to materials vaporized from the target, and evolved condensiblematerials do not readily condense on its surfaces. A substantialdecrease in the contamination of the electron source and a resultinglonger life of the electron source is achieved. Moreover, the tendencyfor negative ions and secondary electrons to be trapped in the electronbeam is substantially reduced by the use of transverse fields. Thisreduces space charge buildup which can detrimentally affect focusing anddeflection.

One example of the use of transverse fields is in connection with themelting of material in an upright crucible. Spalling of condensedmaterials from 0001 surfaces of the vacuum enclosure, and splashing andsplattering of molten material from the crucible, constitutes apotential impairment to satisfactory operation of the electron source.By positioning the electron source underneath the crucible and bydeflecting the electron beam through a curving path of over 180,contamination and shorting of the electron source are avoided and it isunnecessary to utilize splatter shields or similar devices. An electronbeam gun assembly of this type is disclosed in copending applicationSer. No. 464,968, filed June 18, 1965, which is assigned to the assigneeof the present application.

Another example of a use for transverse fields in deflecting an electronbeam is where separately pumped chambers for the target and the electronsource are used and where a small opening communicates between the twochambers. By positioning the electron source out of a line of sight fromthe opening, and by deflecting the electron beam through a curving path,fewer vapor particles passing through the opening, reach the source.Furthermore, there is a reduction in the tendency for negative ions andsecondary electrons to be trapped in the transverse field and impededirection and focus of the beam at the opening. 3

In many instances, including the two just described, it may bedesirable, in addition to deflecting the electron beam through a curvingpath, to prevent divergence or to focus the electron beam so that thebeam converges in a certain region, referred to herein as a focalregion. By causing convergence of the beam into a generally round spotat a focal region which is at or just prior to the surface of thetarget, an efficient transfer of energy will occur between the beam andthe target. By converging the beam at an opening in a wall betweenseparately pumped vacuum chambers, or at an opening in a. fsplatter orvapor shield, the size of the opening may be minimized for betterseparation or shielding.

Heretofore, methods and apparatus for producing and directing electronbeams have not provided satisfactory electron beam convergence for manypurposes. This has proved particularly true where deflection of greaterthan about is desired. Deflection and convergence problems may becompounded by the use of an elongated emitter which produces astrip-like or ribbon shaped beam. Emitters of this type, however, offersome advantages over other types of emitters and it is thereforedesirable that satisfactory focusing of ribbon shaped beams beattainable.

It is a principal object of the present invention to provide an improvedapparatus for producing and'directing an electron beam.

Another object is to provide an improved apparatus for producing anddirecting an electron beam, and which deflects the electron beam througha curving path and converges the beam at a focal region subsequent tothe curving path.

An additional object is to provide an improved electron beam gunassembly which is adapted to be positioned to minimize contact of vaporparticles with the electron source.

A still further object is to provide apparatus for converging anelectron beam at the surface of a target or just prior thereto whileproviding transverse deflection of the electron beam through a curvingpath which is about 90 or greater.

Various other objects of the invention will become apparent to thoseskilled in the art from the following detailed description taken inconnection with the accompanying drawings wherein FIGURE 1 is aschematic elevational end view illustrating an electron beam gunassembly in accordance with the invention;

FIGURE 2 is a cross sectional view taken along line 2-2 of FIGURE 1;

FIGURE 3 is a bottom view of the apparatus of FIGURE 1;

FIGURE 4 is a schematic vertical cross sectional view of a furtherembodiment of the invention;

FIGURE 5 is a schematic vertical cross sectional view of a still furtherembodiment of the invention;

FIGURE 6 is a schematic vertical cross sectional view of still anotherembodiment of the invention;

FIGURE 7 is an end view of the apparatus of FIGURE 6; and 6 FIGURE 8 isa top view of the apparatus of FIGURE Very generally, the apparatus ofthe invention operates to produce an electron beam directed in aninitial path. A non-homogeneous magnetic field is established transverseto the initial path. The electron beam is passed through the transversemagnetic field to be deflected hereby through a curving path. Throughouta substantial portion of the curving path of the beam, the magneticfield has a strength which generally decreases as the distance fromltheinnermost edge of the beam increases. In this portion of the beam path,the field may be shaped so that the electrons toward the outer edge ofthe beam pass through a longer region of the field relative to theelectrons toward the inner edge of the beam.

Turning now to the invention in greater detail, reference is made toFIGURES 1 through 3 wherein a target material 11 which is to be heatedis contained within a crucible 12. A plurality of coolant passages 13are provided in the walls of the crucible 12 and a coolant, such aswater, is circulated therethrough. By cooling the crucible in thismanner, the crucible is constructed of a convenient material such ascopper. Because the crucible is cooled, a solid skull of the targetmaterial forms between the molten pool and the walls of the crucible toprevent any reaction between the material in the molten pool and thecrucible material. The crucible is supported on a suitable base plate14.

The target material 11 and its supporting crucible 12 are all disposedwithin the interior of an evacuated enclosure, not illustrated. Heatingof the material 11 in the crucible 12 is accomplished by means of anelectron beam. As will be explained, the electron beam is of therectangular strip or ribbon shaped type and, in FIGURE 2, itslongitudinal edges are indicated by the dash-pot lines 16 and 17. Theaxis of the ribbon beam is indicated by the dash-pot line 18 in FIGURE2.

The electron beam 16-18 is produced and directed at the target material11 by means of an electron beam gun assembly. The electron beam gunassembly illustrated in FIGURE 1 includes an elongated emitter 19 whichis of a suitable material, such as tungsten, which produces freeelectrons when sufliciently heated. A direct current, the connectionsfor which are not illustrated, is passed through the emitter in order toproduce heating and consequent electron emission. Although notindispensable to the invention, the use of an elongated emitter providesa number of advantages in construction and operation.

The emitter 19 is disposed in the recess 21 in a shaping or focusingelement 22 which is positioned alongside and behind the emitter 19. Theshaping element, as well as the emitter, is maintained at a negativepotential and electrons emitted by the emitter 19 are therefore directedinto a rectangular strip or ribbon shaped electron beam moving out ofthe recess 21. A pair of grounded accelerating electrodes 23 and 24 aresupported by two support plates 26 and 27 and are positioned on eitherside of the beam as it leaves the recess 21. Because the electrodes 23and 24 are positive with respect to the emitter, the electrons in thebeam are accelerated to a relatively high velocity. The emitter ispositioned below the crucible 12 and, as a result, it is protected fromsplatter and vapor which may issue from the surface of the moltenmaterial in the crucible.

In order to direct the electron beam 16-18 from the emitter 19 throughan arc of greater than 180 degrees to the target material 11, and inorder to converge the ribbon shaped beam into a more concentrated shapeat the target for better heating, the beam is passed through anon-homogeneous magnetic field which extends transversely of the path ofthe beam. In the embodiment of FIG- URES l-3, the transversenon-homogeneous magnetic field is established between a pair ofsemicircular pole pieces 28 and 29 of ferromagnetic material. The polepieces are magnetized to be of opposite polarity by connecting the sameto suitably energized solenoids or permanent magnets (not shown). Thetwo semicircular pole pieces 28 and 29 are supported on either side ofthe beam by suitable means (not shown) so that the straight edgesthereof lie in a vertical plane extending transverse to the axis of theinitial beam path and slant toward each other from their lower ends atan angle B from the vertical for a purpose hereinafter explained. Theirlower ends are disposed below the emitter and their tops are disposedabove the surface of the target. The opposed faces of the pole pieces 28and 29 are flared outwardly with respect to each other, for a purposedescribed hereinafter. Suitable means (not shown) for cooling the polepieces, is provided.

This positioning of the pole pieces 28 and 29 provides a field whichdecreases in intensity from the straight edges and bows outwardly towardthe curved edges due to the flare of the pole pieces and which increasein intensity from bottom to top adjacent the straight edges of the polepieces. The field deflects the beams 16-18 over about 225 through agenerally planar arcuate path. The outwardly bowed flux lines preventthe electrons in the beam from diverging in a direction normal to theplane of the beam as the beam passes through the field. The electronstoward the inner edge 16 of the beam, upon entering the field and untila position adjacent their exit from the field, pass through a strongerfield than the electrons toward the outer edge of 17 of the beam.However, since the path of travel of the electrons increases as theirdistance from the inner edge 16 increases, the electrons toward theouter edge 17 are under the influence of the field for a longer path oftravel, preventing divergence or causing convergence according to thevariance in path lengths.

In FIGURES 1-3, the angle of the flare of the pole pieces 28 and 29 ismade such that the beam remains substantially of the same width throughthe greater part of its path of travel through the field. For adeflection greater than 180, the angle of flare, as indicated by A inFIGURE 3, is preferably between about 50 and 55 to provide a constantwidth beam throughout most of its travel through the field. At and nearthe region in which the beam emerges from the non-homogeneous magneticfield, because of the angle B of the straight edges of the pole pieces,the outer edge 17 of the beam is in a stronger region of the magneticfield than the inner edge of the beam. This causes the electrons towardthe outer edge 17 to be deflected more than those toward the inner edge16, thus resulting in a convergence or focusing of the beam exiting fromthe field. The angle of the straight edges of the pole pieces isselected to provide the desired convergence of the beam. To provide agenerally circular beam at the target, the angle B is made about 5.

The particular arrangement utilized in the apparatus of FIGURES 1through 3 is a convenient way for achieving convergence at the surfaceof the molten material 11 in an electron beam furnace system. The shapeof the pole pieces is not necessarily critical to the invention,however, their relative spacing and angular relationship determines thevariation of the field strength. For example, a substantially roundimpact area is achieved at the surface of the melt about 3% inches fromthe straight edges of the pole pieces for an electron beam power of 13kilowatts. The emitter is 4 inches long, and its center about 4 /2inches below the melt surface. The upper ends of the pole pieces areseparated by a 4-inch gap and diverged along the straight edges about 10and flared about 55. The pole pieces are 23 inches along their straightedges and the maximum width of the pole pieces 23 inches. A fieldstrength which increases from about 10 gausses at the region of beamentry to about 20 gausses at the region of beam exit is satisfactory.Other field configurations may 'be established depending upon theparticular beam deflection characteristics desired.

Referring now to FIGURE 4, it may be seen that the apparatus of theinvention is of advantage in a situation wherein the emitter extendstransversely of the plane of the arcuate beam path, rather than lying insuch plane, as was the case in the embodiment of FIGURES '1 through 3.As previously mentioned, space charges and ambient magnetic fieldswithin the vacuum environment often produce a tendency for the electronsin the beam to diverge. This divergence can have a detrimental efiect onthe heating properties of the beam 25. By passing through a fieldof thesame type as in FIGURES 1 through 3, this tendency to diverge iscorrected, as illustrated by the dotted lines in FIGURE 4.

FIGURE 4 may also be taken as representative of a situation wherein aspot source of electrons, rather than an elongated source, is utilized.As is the case with an elongated source, a spot source also produces abeam wherein the tendency is for electrons to diverge. Correction ofthis is achieved as explained in connection with the elongated sourceand with reference to FIGURE 4.

Referring to FIGURE 5, it will be seen that the apparatus of theinvention is of advantage in connection with two electron sources oremitters, rather than one. Occasionally, such as where substantialelectron beam power is desired, it is of advantage to utilize twoelectron sources, rather than one. These sources are illustrated andindicated in FIGURE 5 at 46 and 47. The beams 48 and 49 produced therebyhave their axes indicated by the dotted lines. It will be seen that thebeams are converged into a desired relationship at the surface of themolten material 11. In addition, the magnetic field established causeconvergences or focusing of the electrons in each beam as was describedin connection with FIGURE 4.

Referring now to FIGURES 6 through 8, an embodiment of the invention isillustrated which, under some circumstances, provides additionalversatility. Two pairs of pole pieces 51, 52 and 53, 54 are used. Theelectron source 56 is identical with that of FIGURES 1 through 3,providing a ribbon shaped electron beam 55. The pole pieces 51 and 52are flared outwardly, but their straight edges are substantiallyparallel with each other and with the plane of the arcuate path throughwhich the electron beam is deflected. Thus, as viewed in FIGURE 6, thestrength of the filed diminishes from right to left in the samedirection as the path of the electrons as they leave the electron gun orsource 56. The strength of the field between the pole pieces 51 and 52is established to cause the beam to be deflected through substantially90.

After leaving the field established between the pole pieces 51 and 52,the beam passes across a gap and then enters the field establishedbetween the pole pieces 53 and 54. It may be noted that the pole pieces53 and 54 not only are fiared outwardly in the same inanner as the polepieces 51 and 52 but have their straight edges disposed at an angletoward each other. Thus, the radius of curvature of the electrons in thebeam in the field established between the pole pieces 53 and 54decreases as they move toward the upper ends of the pole pieces 53 and54.

By utilizing two separate pairs of pole pieces in the manner justdescribed, the strengths of the non-homogeneous fields establishedtherebetween can be varied on an individual basis to select a desiredrelation therebetween and effect a desired deflection and focusingcharacteristic to the beam. Moreover, the electron source can bepositioned a substantial distance below the level of the molten material11 and the upper pair of pole pieces 53 and 54 positioned at thenecessary level to deflect the beam from a vertical path onto thetarget.

It has been found that the utilization of non-homogeneous magneticfields, as described herein, afiords particular advantages in electronbeam furnace systems. These advantages include the ability to effect adesired deflection of the beam from a source positioned below the targetand a desired focusing of the beam at the surface of the moltenmaterial.

Various other embodiments of the invention, in addition to those shownand described herein, will be apparent to those skilled in the art fromthe foregoing description and accompanying drawings. Such otherembodiments, and modifications thereof, are intended to fall within thescope of the appended claims.

What is claimed is:

1. Apparatus for producing and directing an electron beam, comprisingmeans for producing an electron beam directed in an initial path, meansfor establishing a magnetic field transverse to the initial path fordeflecting the electron beam through a curving path, said fieliiestablishing means including a pair of pole pieces positioned onopposite sides of the beam path, said pole pieces having surfacesoriented with respect to each other and to the initial path so thedistance between said surfaces increases with the distance from 'saidbeam production means in the direction of the initial path to produce anon-homo geneous transverse magnetic field which has a strength whichfor a greater part of the path of travel of the beam in the fielddecreases in the plane of the curving path from the inner edge of thebeam toward the outer edge.

2. Apparatus according to claim 1 in which said pole pieces are ofsufficient size that the electronbeam remains in the region directlybetween them throughout the curving path so that the electrons towardthe outer edge of the beam pass through a longer region of the fieldrelative to the electrons toward the inner edge of the beam. a

3. Apparatus in accordance with claim 1 in which said surfaces of saidpole pieces are oriented so that the distance between them increasesfrom the outer edge of the beam to the inner edge at the exit of theelectron beam from the field, whereby the field is of greater strengthat the outer edge of the beam than at the inner edge at the exit.

4. Apparatus according to claim 1 in which said field establishing meanscomprise a pair of generally flat pole pieces positioned on oppositesides of the curving portion of the beam path.

5. Apparatus according to claim 1 in which said field establishing meanscomprise a pair of generally semicircular spaced discs each having asubstantially straight edge positioned toward said beam producing means,said discs having an angle of divergence along the straight edgesthereof opening toward the region of entry of the electron beam into themagnetic field, and having a further angle of divergence outwardlytoward the midpoint of their curved edges.

6. Apparatus according to claim 1 including means for establishing asecond transverse non-homogeneous magnetic field prior to thefirst-named magnetic field, said second magnetic field establishingmeans including a pair of second pole pieces positioned on oppositesides of the beam path, said second pole pieces having surfaces orientedwith respect to each other and to the initial path so that the distancebetween said surfaces increases with the distance from said beamproducing means in the direction of the initial path such thatthroughout a substantial portion of the curving path of the beam thesecond magnetic field has a strength which decreases as the distancefrom the innermost edge of the beam increases.

References Cited UNITED STATES PATENTS 2/1967 Muller.

2/1968 Takizawa.

U.S. Cl. X.R. 139; 219-121

